This invention pertains to flywheel energy storage systems and more particularly to a flywheel power source with a heat transfer system for passively cooling the stator of the generator. The flywheel power source achieves an extended generator life and increased power capability with lower costs and higher reliability than previous flywheel systems.
Flywheels have been used for many years as energy storage devices. They have often been used as power smoothing mechanisms for internal combustion engines and other kinds of power equipment. More recently, flywheels have been recognized as a very attractive energy storage technology for such electrical applications as uninterruptible power supplies, utility load leveling systems, satellites and electric vehicles.
Modern flywheel energy storage systems convert back and forth between a spinning flywheel""s rotational energy and electrical energy. A flywheel energy storage system includes a flywheel, a motor/generator, a bearing system and a vacuum enclosure. The rotating flywheel stores the energy mechanically; the motor/generator converts between electrical and mechanical while the bearing system physically supports the rotating flywheel. High-speed flywheels are normally contained in a vacuum or low pressure enclosure to minimize aerodynamic losses that would occur from atmospheric operation while low speed systems can be operated at atmosphere.
In almost all UPS applications, whether quick discharge type (power ride-through), where discharge time is measured in seconds, or long-term discharge type (power back-up), where discharge time is measured in hours, flywheels directly compete with electrochemical batteries. Power ride-through systems are usually for higher power ( greater than 50 kW) applications that can be coupled with a generator set. The flywheel supports the load for the first 30 seconds to 1 minute of the electrical interruption period until the generator can get up to speed or the electrical interruption has ceased. Back-up systems are usually for lower power ( less than 50 kW) applications where the flywheel is expected to support the load for the duration of the electrical interruption period. Two key advantages of flywheels used for electrical energy storage over electrochemical battery systems are longevity and reliability. Electrochemical batteries, in particular, lead acid batteries, have short lifetimes, between two and seven years depending on operating conditions. These batteries require periodic maintenance and can fail unpredictably. In contrast, flywheel energy storage systems are expected to have lifetimes of at least twenty years and it is desirable not to require any maintenance. Such capability can offset the higher initial cost of the flywheel system over batteries by actually becoming cheaper when considered over the life.
All designs of flywheel motor/generators have electrical coil windings for conversion between electrical and magnetic energy used to apply torques to the rotating flywheel. The lifetime and reliability of the motor/generator is directly related to the dielectric insulator life of the insulation on the coil wires. After long-term operation, the dielectric strength of the insulation breaks down causing arcing and shorting. The lifetime of the insulation has been shown to follow a form of the Arrhenius Law with temperature. For a given insulation, the life is cut in half for every 6xc2x0 C. operating temperature increase. Besides the loss of dielectric breakdown and insulator strength, high temperatures can also physically melt the insulation. One solution to increasing the life of the motor/generator is to use heavier and or higher temperature insulation. Using heavier insulation results in less space for the copper conductors, increasing their resistance and therefore the heat generated by the motor/generator. Heavier insulation also reduces the heat transfer from the coils. A physically larger motor/generator could be used to keep the higher efficiency with the heavier insulation but this significantly increases the cost of the motor/generator. This is especially true in flywheels where the high operating speeds (up to 40 krpm, 666 Hz) requiring use of expensive, very thin laminations to stack up the stator for reduction of eddy currents. Despite the increased costs of using a larger motor/generator, it is not a guaranteed solution to the problem. Size is usually limited by the stress capability of the rotor portion. Likewise, the stator laminations themselves and other metal components, if used, can be a larger source of heat generation through eddy current and hysteresis losses. No changes in the coil insulation can affect this generated heat. Further exacerbating the heat generation and removal from stators in flywheel power sources is that most designs place the stators inside the vacuum container for efficient magnetic coupling. This makes cooling and heat transfer from the flywheel stator much more difficult. High temperatures in the motor/generator can also have an added negative effect on the flywheel system in that the outgassing in the vacuum is strongly related to temperature. Insulation materials, in particular, already exhibit high vacuum outgass rates even at room temperature. Substantial outgassing of stator winding insulation can degrade the vacuum and require costly vacuum renewal steps or shorten the operating life of the flywheel power source.
The second method to extend the life of the flywheel system motor/generator is to actively keep the stator cool. With high power flywheel systems, dissipation of up to several kilowatts of heat power can be necessary. To date, such systems have employed pumped cooling systems. This method is effective in removing the heat but the weak link of the system reliability and longevity becomes the pump. To remove the heat, the liquid is also pumped outside the vacuum chamber. This requires the use of expensive, complicated and potentially unreliable fluid connections and fluid feedthroughs. One such flywheel system using forced liquid convection cooling is shown in FIG. 1. The flywheel power source 30 is comprised of a high speed flywheel 31 that rotates inside an evacuated chamber 35 within a container 34to reduce aerodynamic drag. Flywheels can be constructed of composite materials such as carbon fiber/epoxy or of metals such as steel. As shown, the flywheel 31 is constructed with a carbon fiber epoxy rim 32 mounted on a solid metal hub 33. The flywheel 31 has upper and lower shafts 36 and 37 for journaling the flywheel and for attachment of a motor/generator 40. The flywheel is supported for rotation using upper and lower bearings 38 and 39. Flywheel systems typically employ mechanical bearings, magnetic bearings or a combination of the two types.
The motor/generator 40 is comprised of two portions: a rotor 41 that is attached to the flywheel, and a stator 42 that is stationary. The rotor can be a reluctance type motor/generator gear or alternatively permanent magnet type as shown. Any type of motor/generator can be used as long as it does not require brushes that wear. The rotor 41 is surrounded by and cooperates with the stator 42. The stator contains internal electromagnetic coils (not shown) for electrical-mechanical energy conversion with the rotor 41. In this case, the stator also includes internal laminations (not shown) for efficient completion of the motor/generator magnetic circuit. The stator 42, which is sealed inside the vacuum 35 to prevent a loss of vacuum, is cooled by pumping coolant through it. The coolant enters and exits the stator 42 and vacuum container 34 through inlet and outlet feedthroughs 43 and 44. A separate, and in this case external, pump and or cooler pumps the fluid. The forced convection from fluid is a highly effective method for removing the heat from the motor/generator. Unfortunately as described previously, this type of flywheel system has reduced reliability, and is complicated and expensive.
The invention provides a flywheel power source that is capable of high power operation with long life and whose cooling has high reliability with low costs. The realization of the invention is made possible by consideration of the operation of flywheel systems. Unlike previous flywheel systems with motor/generator cooling, the invention takes into account the actual service conditions of the flywheel system to provide more appropriate cooling. In high power flywheel systems, the highest power load is typically required only for a short period of time between very long low power conversion periods. This high power period is the result of discharging the flywheel from a fully charged state in a relatively short period of time. The actual damaging effects to the windings occur at the highest temperatures during this peak power operation. In almost all high power flywheel UPS applications, only the discharge of the flywheel is at very high power and the charging can be done at low power. If the flywheel is used for ride-through until an auxiliary generator set can turn on, the generator can be left running until both the power is restored and the flywheel is fully recharged. A high power recharge capability is not typically required. High power recharging from line power can also overload electrical supply lines in some cases and is not desired. Whether high power recharging is required for specific application or not, the period of time between high power operation can be days, weeks or months. When the flywheel is rotating at full speed and the system is fully charged, heat generation from losses is usually negligible. Only a small amount of power is required to maintain full speed in a well designed system. The goal of the flywheel cooling system is therefore most importantly to limit the maximum temperature and the duration of higher temperature during high power discharging, and optionally during charging cycles. Continuous pumping of cooling fluid is both unnecessary and it also reduces the longevity and reliability of the overall flywheel power source and increases the energy costs of operating the system.
The flywheel power source of the invention maintains cool generator operation passively by surrounding the generator stator coils with a sealed and partially filled liquid vessel in which heat energy from the stator is transferred to the liquid. The liquid absorbs the finite amount of heat energy generated from individual high power discharge cycles. Although forced convection by using a pumped fluid can provide as much as ten times higher heat transfer coefficients, it has contrarily been found that the maximum temperature of a flywheel stator can be sufficiently limited passively. Instead of staying at a near constant temperature, the liquid in the sealed vessel increases in temperature some amount, however the temperature of the stator is kept acceptable. One key to this is that the flywheel only stores a given amount of energy and therefore the amount of heat energy that can be generated and has to be absorbed is limited. The coolant absorbs the heat energy from the high power operation period and then that amount of energy is dissipated from the liquid over a long period of time. The cooling system of the invention would not be applicable for applications that require continuous or long-term high power operation but it uniquely matches the operation of flywheel power sources. The vessel is preferably substantially large to hold enough liquid for adequate cooling and heat energy absorption. The losses from the generator and the amount of heat energy to be transferred to the liquid is also reduced by winding the stator with multiple individually insulated conductor wires, or Litz wire, coils. By being only partially filled with the liquid, the vessel prevents rupture due to thermal expansion and increased pressure when heat energy is absorbed. The vessel can also enclose the stator laminations, if used, to cool them and in flywheel systems using a single combined motor/generator, cooling is also provided during recharging.
The passive cooling of the flywheel generator can work by two different embodiments. In both cases, heat is transferred from the stator to the liquid through both conduction and radiation. In one embodiment, the liquid inside the vessel efficiently absorbs much of the heat by natural convection and the heat capacity of the liquid. Natural convection works by the heated liquid having a different density than the cool liquid, causing it to flow and hence cool the motor/generator. Low viscosity of the liquid is essential to the efficacy of the process. The liquid has a low viscosity, preferably less than 200 mm2/sec and more preferably less than 20 mm2/sec at 40xc2x0 C., for increased flow and better cooling. Because the vessel is sealed, no liquid escapes and needs to be replaced. The sealed vessel also prevents contamination of the vacuum from the fluid outgassing. Sufficient room without liquid is preferred inside the vessel to prevent excessive pressure from evolved gas that may occur over time and from thermal expansion of the fluid. After a short amount of time and before the next high power charging or discharging of the flywheel uninterruptible power supply, the vessel itself and internal liquid cools, resetting the generator or motor/generator for the next cycle. Many possible types of fluids can be used including water, oils, and others preferably nonconducting, noncorroding and with low viscosity. If water is the liquid, it is preferably pure and is not used when exposed corrosion prone laminations are in the vessel. The generator wires exit the sealed fluid vessel through use of electrical feedthroughs or other hermetic seals. The vessel itself is preferably constructed of a low electrical conductivity material or is made sufficiently thin to reduce magnetic induced losses. In one embodiment, dividers or a flow separator wall is included inside the vessel so as to increase the natural convection flow velocity over the stator and its effective cooling. In another embodiment, the level of the liquid can be contained to a height significantly higher than the stator so as to increase buoyancy and flow velocity. The vessel can also have a portion external to the flywheel vacuum container for cooling the oil.
The second embodiment for stator cooling is through the added energy absorption from heat of vaporization of the liquid. It is known that energy transfer through boiling of a liquid can be extremely high and it has been found that this mechanism can be employed to limit the high temperature experienced by a flywheel power source stator. In this case, radiation, conduction and natural convection are also occurring but an added large amount of heat is also absorbed by the transformation of the liquid to a gas. The vessel contains a fluid that has a relatively low boiling point inside the vessel. Some liquids that can be used include water, solvents like alcohol and fluorocarbons. These conveniently have low boiling temperatures at atmospheric pressure, however other liquids with different boiling points can also be employed by adjusting the internal pressure inside the vessel before sealing. When the flywheel power supply is intermittently charged or discharged at high power, the electrical windings become hot due to the resistive heating. If a ferromagnetic core is included in the motor/generator, it can also become hot due to eddy current and hysteresis losses. The liquid inside the vessel efficiently absorbs much of the heat by local vaporizing. Because the vessel is sealed, no vapor escapes and needs to be replaced. In this case, increased head space above the liquid level is needed inside the vessel to prevent excessive pressure. After a short amount of time and before the next high power cycle of the flywheel system, the vessel itself cools and the gas condenses, resetting the motor/generator for the next cycle. Some gas formed also condenses immediately during the discharge cycle and it transfers heat energy to the remaining liquid. By this mechanism, the stator is effectively cooled and much energy is distributed to the heat capacity of the liquid, reducing the internal pressure developed in the vessel. The bubble formation and rising further enhances free convection flow and cooling. Careful choice of the liquid, internal vessel pressure and the high power cycle losses is required in the vaporization cooling design. Care must be taken to prevent exceeding the critical heat flux of the liquid, in which case heat transfer becomes reduced by the increased gas formation actually insulating the heat generating surfaces. However, as the pressure in the vessel increases, so does the boiling temperature of the liquid because of the sealed container and in some cases this can be beneficial in preventing transition from nucleate to film boiling.